yffi: observer callbacks do not supply the transaction origin

[britannio]

I'm working on a y-crdt port to Dart (https://github.com/britannio/y_dart) but the lack of an origin in observe callbacks makes it more challenging to implement bindings for storage and text editing.

My current solution introduces a transaction_origin_data() function copied from YUndoEvent.new https://github.com/britannio/y_dart/blob/044c6d5f1022fcbbe7d21d342297fa24a07645a6/rust/src/lib.rs#L510-L544